1. Field of the Invention
The present invention relates to a process for the treatment of heavy oils, including crude oils, vacuum residue, tar sands, bitumen and vacuum gas oils, using a catalytic hydrotreating process. More specifically, the invention relates to the use of catalysts in series in order to prolong the life of the catalyst. In another embodiment, the presence of light hydrocarbon components in conjunction with the heavy oils is used for improved treatment of the heavy oils utilizing moderate temperature and pressure.
2. Description of the Related Art
Hydrotreating is useful for the purpose of improving heavy oils. The improvement can be evidenced as the reduction of sulfur content of the heavy oil, an increase in the API gravity of the heavy oil, a significant reduction in the metal content of the heavy oil, or a combination of these effects.
The availability of light sweet crudes is expected to diminish in the future as the production of oil becomes increasingly difficult and greater reliance is placed on tertiary and enhanced recovery techniques. Heavier crudes and sour crudes will take on greater importance in overall hydrocarbon production and the upgrading of such crudes into fuels will become increasingly important. In addition to the decreasing quality of the crudes and their derived heavy oils, specifications for on-road and off-road fuel will become increasingly more stringent, driven by environmental legislation around the world. A greater emphasis on upgrading and degree of hydroprocessing can be expected in the refining industry.
One of the main limiting factors for hydroprocessing units is the deactivation of the hydroprocessing catalysts. As the heavy oil feedstock being treated becomes heavier, i.e. has a lower API Gravity, the complexity of the molecules increases. This increase in complexity is both in the molecular weight and also in the degree of unsaturated components. Both of these effects increase the coking tendency of the feedstock, which is one of the main mechanisms of deactivation of the catalyst. Another aspect of feedstock leading to deactivation of catalyst is metal content present in the heavy crude. These metals are normally present in the form of porphyrin type structures and they often contain nickel and/or vanadium, which have a significant deactivating effect on the catalyst. Similar to coking tendency, the metal concentration of the heavy oil feedstream increases with decreasing API gravity.
Any pre-refining of crude oil would provide a significant advantage for downstream process units.
As the refining industry increasingly processes higher sulfur, lower API crudes, catalyst deactivation will become a critical path problem, decreasing the on-stream cycle length and therefore increasing the cost of processing, negatively impacting process profitability. Advances in the treatment of heavy oil with respect to a reduction in catalyst deactivation will therefore be of paramount importance to the refining industry in future years.
Global diesel demand is forecasted to increase in the coming years due to the dieselization trend, equaling global demand for gasoline in the near future and surpassing this demand thereafter. A shift in product slate is occurring. The inherent content of the gas oil in crude oils is limited and conventional, expensive conversion techniques such as hydrocracking are required to increase the diesel yield by conversion. There is a need to provide a process for heavy oils that will increase diesel production in a cost-effective manner to meet market demands. There is a need to provide a process that minimizes capital expenditures necessary while meeting the product specifications.